Seedless Plants Notes

Land Plant Origins

  • Common Ancestry: Green algae and land plants share a common ancestor, dating back approximately 1 billion years ago.
  • Evolutionary Lineage:
    • Archaeplastida: The group from which green algae and land plants are derived, including Chlorophytes and Charophytes.
    • Streptophytes.
    • Embryophytes: Land plants.
    • Tracheophytes: Vascular plants.
    • Euphyllophytes.
    • Spermatophytes: Seed plants.
  • Evolutionary Tree:
    • Charophytes -> Bryophytes.
    • Lycophytes.
    • Ferns.
    • Gymnosperms -> Angiosperms.
  • Key Adaptations:
    • Vascular tissue.
    • Stomata.
    • Sporophyte-dominant life cycle (diplohaplontic).
    • Ovules and pollen leading to seeds.

Charophytes

  • Description: Multicellular, photoautotrophic protists.
  • Significance: Closest living relatives to all land plants.
  • Habitat: Primarily freshwater environments.
  • Relevance: Highlights the continued requirement for freshwater in land plants.

Issues Living on Land

  • Charophytes live in freshwater, while terrestrial habitats present several obstacles.

1. Desiccation

  • Problem: Loss of water.
  • Solutions:
    • Waxy cuticle: Impermeable layer to prevent water loss but limits gas exchange.
    • Stomata: Openings to allow gas exchange.
    • Mycorrhizal associations: Symbiotic relationship between plants and fungi, where fungi help make nutrients available.

2. Transport

  • Problem: Movement of nutrients.
  • Solutions:
    • Xylem: Transports water.
    • Phloem: Transports food and hormones.

3. UV Radiation

  • Problem: Increased chance for mutations.
  • Solution:
    • Minimize mutation effects via diploidy.
    • Plants tend to have large genomes, and many species are polyploidy.

Haplodiplontic Life Cycle

  • Both diploid and haploid life stages undergo mitosis.
  • Sporophyte: Diploid, produces spores via meiosis.
  • Gametophyte: Haploid, produces gametes via mitosis.
  • Sporangia: Spore-producing cells.
  • Gametangia: Gamete-forming cells.
  • Reduction of gametophyte stage correlates with increased terrestrial specialization.

Bryophytes

  • Dominant gametophyte generation and earliest land plants.
    • Tied to water: Limited ability to regulate internal water and tolerate desiccation.
    • Limited nutrient transport through cells (no vascular tissue).
    • Photosynthetic gametophytes: Sporophytes directly attached to gametophytes.

Three Main Groups:

1. Liverworts (Phylum Hepaticophyta)
  • Flattened gametophytes with liver-like lobes.
  • Rhizoid: Root-like structures that aid in absorption and anchor to the substrate.
  • Air chambers for gas exchange but lack stomata, so they cannot close.
2. Mosses (Phylum Bryophyta)
  • Leaf-like gametophytes with rhizoid and midrib (stem-like axis).
    • Sporophyte (2n2n).
    • Gametophyte (nn).
  • Rhizoid: Root-like structures.
  • Midrib: Stem-like axis.
  • "Leaf": One cell thick, photosynthetic.
  • Capsule: Sporangia with stoma; produces spores.
Moss Reproduction
  • Gametophyte is the dominant life stage; sporophyte is non-photosynthetic and attached to the gametophyte.
  • Archegonia: Female gametophyte.
  • Antheridia: Male gametophyte.
  • Sperm is flagellated and swims to the egg in water for fertilization.
    • Archegonium: Single haploid egg via mitosis.
    • Antheridium: Many haploid sperm via mitosis.
    • Fertilized zygote develops within the archegonium and forms the sporophyte.
3. Hornworts (Phylum Anthocerotophyta)
  • Sister-group to tracheophytes (vascular plants); possesses stomata for gas exchange.
  • Morphologically similar to liverworts, but the sporophyte is a photosynthetic horn.
  • Sporophyte has hornwort stoma.